This invention related to pumps, and particularly to a pump and pumping method for pumping both relatively high volumes at low pressure and also relatively lower volumes at higher pressures.
Many pump applications require a pump to move relatively large volumes of liquids, to fill a reservoir for example, and also apply a relatively high pressure to the fluid once the reservoir is filled. For example, oil well blowout preventer stacks must be tested periodically by plugging the wellhead to which the stack is connected, closing the blowout preventer mechanism to be tested, and then filling the area of the blowout preventer with a liquid and applying the desired test pressure. The pump used to fill the blowout preventer must be capable of moving a substantial volume of liquid to initially fill the blowout preventer stack and must also be capable of applying test pressure, which may be 5,000 psi or more.
However, pumps have heretofore been designed to operate in either a high volume, low pressure mode or a low volume, high pressure mode. The reason for this is that, mechanically, a given input power may be used to move a given volume of liquid at a certain pressure or a relatively lesser volume of liquid at a relatively higher pressure. Thus, in applications requiring a pump to move large volumes of liquid and also apply a high pressure, some compromise was required. The compromise often meant using a low volume, high pressure pump to move a large volume of liquid over an excessively long period of time. Even where a pump could switch from low pressure, high volume operation to high pressure, low volume operation, the pump had to be manually switched between modes, which was time consuming and required that the pump operator have some minimum level of training.